Photonic crystals exhibit exciting opportunities for controlling light. This can be utilized in optical waveguides, telecommunication devices, chemical and biological sensing and solar cells. However, functional devices require tuneable photonic crystals and ultimately structures that allow switching. Therefore inorganic-polymeric hybrid particles were developed, which offer new opportunities in the design of nanostructured materials since each component can be removed selectively. The presentation describes the synthesis of silica-polymer core-shell particles, absolutely uniform in size and in architecture. They were used as building blocks for colloidal crystals which served as templates for titania or tin disulfide inverse opals. Due to the inorganic-polymeric core-shell structure of the spheres, removing the polymeric shells by calcination yields the structure of Double Inverse Opal Photonic Crystals (DIOPC): The ordered, 3D array of air spheres in a high refractive index backbone is a host for movable silica spheres which behave as weakly scattering objects. The optical properties were determined by reflectivity measurements. Infiltration with liquids masks the silica-spheres optically, therefore introducing a transition of a diffusive scattering material to a selective reflection of specific wavelengths as known from ordinary inverse opals. This experiment simulates an order-disorder transition, induced by a collective shift of the random distributed silica-spheres into one specific position in the pores. The potential of switching a complete band-gap by a collective shift of the spheres to specific positions in the pores of the DIOPC is demonstrated by computational analysis.
Materials with a periodically modulated refractive index, with periods on the scale of light wavelengths, are currently attracting much attention because of their unique optical properties which are caused by Bragg scattering of the visible light. In nature, 3d structures of this kind are found in the form of opals in which monodisperse silica spheres with submicron diameters form a face-centered-cubic (fcc) lattice. Artificial opals, with the same colloidal-crystalline fcc structure, have meanwhile been prepared by crystallizing spherical colloidal particles via sedimentation or drying of dispersions.
In this report, colloidal crystalline films are introduced that were produced by a novel technique based on shear flow in the melts of specially designed submicroscopic silica-polymer core-shell hybrid spheres: when the melt of these spheres flows between the plates of a press, the spheres crystallize along the plates, layer by layer, and the silica cores assume the hexagonal order corresponding to the (111) plane of the fcc lattice. This process is fast and yields large-area films, thin or thick.
To enhance the refractive index contrast in these films, the colloidal crystalline structure was inverted by etching out the silica cores with hydrofluoric acid. This type of an inverse opal, in which the fcc lattice is formed by mesopores, is referred to as a polymer-air photonic crystal.